A giant impact from an asteroid or comet can ruin your whole day. Or year. Or, if you’re a dinosaur, your existence.

So astronomers do what they can to understand this menace from space. We look for rocks on orbits that intersect ours, we think about ways of moving them out of the way should we find one, and we also think about the record we do have of past impacts to see what we can learn from them.

There are about 180 impact craters known on our planet, ranging from tens of millennia in age to billions of years. They also vary in size from a few kilometers across to monsters so big they can only be detected from space. Sometimes it’s hard to measure their size (they can have multiple concentric rings, or be underground — covered up due to extreme age — making definite sizes hard to figure out) or hard to get their age. But we do have some statistics on them, and there have been many studies about them.

A big question is: are impacts periodic? That is, do they happen with some repeating period? If so, then there must be some astrophysical cause: a giant planet in the outer solar system, for example, that shakes loose comets every 50 million years, or the Sun passing near another star. This has been studied, and all kinds of periods have been found in the data. I’ve always been a little skeptical of them, since the data are sparse. And now it looks like my thoughts are being supported: a new study finds no such pattern in the ages of craters, and concludes all the periods found previously are probably due to errors in the analyses.

The difference is that the author, Coryn Bailer-Jones, used Bayesian statistical methods. This is different than standard statistics, and is less prone to bias due to uncertainties in age and size of craters. In using standard statistics, clusters in crater ages can always be found, but it’s hard to know if that’s just a random clump or has an actual physical cause — like flipping a coin 10 times and having it come up heads 5 times in a row. It’s unlikely, but how do you know if it’s coincidence or not? Bayesian methods circumvent that issue.

Bailer-Jones found that when the uncertainties are treated properly, the periodicity of impact events seen in previous studies vanishes. We’re not bombarded every 60 million years (or whatever); it was simply an artifact of the way the math was being done. And while I won’t go so far to say that this closes the book on periodic impacts, it makes it look a lot less likely*.

There were two other results found that I think are particularly interesting, too. One is that with huge craters, ones bigger than 35 km (21 miles) across — in other words, more likely to cause extinction-level events — there is no trend in the rate of impacts over the past 400 million years. That is, we’re getting hit just as often now as we were back then, which is to say, not very often.

The second is even more interesting: for craters of all sizes (not just big ones), there is a trend of increasing rate over the past 250 million years. Are we getting hit more by smaller objects now than we were in the past?

Not necessarily! That’s one explanation, of course, but another is that smaller craters are more easily eroded over time. We don’t see a lot of old ones because they fade away due to wind, water, earthquakes, and so on. That makes it look like we’re seeing more now, but that may be an illusion; once erosion is taken into account the rate of impacts looks like it’s steady once again. However, I’ll note that when you look at the Moon there’s evidence that impacts have increased over time, so these conclusions are by no means firm. After all, we’re basing them on a relatively small number of craters, but it’s what we’re stuck with.

So it looks like impact rates from space are not periodic, at least not over the past 400 million years or so. We’re not "due" for one any time soon. And perhaps more importantly, given how some media tend to report these things (cough cough), there’s no solid evidence that we’re more at risk now than sometime in the past. I know a lot of people worry about these things, so I want to be clear on that.

And the good news is, as I pointed out recently, when their number came up dinosaurs didn’t have a choice to live or die. We have a space program, so the choice is ours.

* Note that this study was only using impact from space. There are studies that do show periodic rates of mass extinction on Earth. If true, these are due to other things, and not impacts. One interesting event may be due to the Sun’s orbit around the Milky Way. I find this idea compelling, though the evidence is still too sparse to be reliable.

How can an impact be so large it is only detected from space? Do you mean it is more easily (or first) detected from space? It seems like it’s always easier to detect things on Earth from Earth, than from space.

Heh. So our sudden annihilation by a giant meteor will be essentially completely random! Yay!

Just kidding. I mean, that *is* better than finding out there’s a regular pattern and we’re bumping up against when the next one would supposedly occur. It’s just that in as much as I worry about us getting smoked by a big rock or snow ball from space (which is to say, enough that I think we should be devoting more resources to finding potentially dangerous objects and working on implementing possible solutions), this news isn’t all that comforting.

A comforting conclusion would probably be that there is a period, and we’re not “due” for another hundred million years. If we can’t get ourselves off this rock in that amount of time (which itself presumes we wouldn’t have died off due to non-impact-related reasons), then Oh Well we had a good run.

This actually sounds like good news in that we can present the data and instead of the powers that be saying “based on past impacts we don’t have to worry for X number of years so get back to us later”. Since the period of impactors is random, we need not delay putting in effect some means of redirecting a body in a impact lane towards our planet. Be a good secondary goal for the use of the same technology that NASA has wanted for the exploration of the asteroids.

In any case it seems that flood basalt eruptions have a better track record of occurring suspiciously close to mass extinctions than large impacts do. I wonder if it would be possible to detect the signature of a flood basalt eruption in an exoplanetary atmosphere…

Novice thought… but wouldn’t the rate, age and periodicity of impacts on the moon be indicative of the same on the earth, proportional to the mass or gravitational attraction? This may require more extensive data on lunar craters than we have, but we surely have more on that than we do on terrestrial craters.

With a good long range detection system the removal of an object from our path gets cheaper, the more time you have to apply a force on an object, the smaller the force needs to be thus smaller the engine. Of course, big time politics doesn’t understand doing things smaller…..

The next time I meet a chicken, I’ll thank it in behalf of his distant relatives for saving mine.

“when their number came up dinosaurs didn’t have a choice to live or die. We have a space program, so the choice is ours”

I don’t understand what is intended by the word “choice” here.

As a matter of logistics, only a near-infinitesimally small fraction of a percent of the 7 billion or so members of the humans species would have the least scope for choice in response to a massive impact on anything like the scale of the one associated (rightly or wrongly) with the demise of large dinosaurs. Exceptional wealth would not appear to provide any assurance of fitness to handle the demands of exit into and travel in space for any significant periods of time and distance; what level of fitness for space travel would be necessary to sustain human life to the point of being able successfully to colonize elsewhere in the universe is not known; we do not know towards where it may be safe to travel for the purposes of colonization; if we knew where such places were, it’s entirely unknown whether just the descendants of the even tinier number of those selected would make it; and so on.

There is a big gap between saying w”e have a space program” and actually having a program able to overcome the huge number of technical barriers to human space travel for colonization. It seems to me we are closer than not to having a child’s little red pull wagon and calling that a “space program”.

@ 1 Steven,
It can be easier to see the really big ones from space. It can be hard to make measurements that are accurate enough over such a wide area. It can also take measuring devices that need to be up above the surface. From space doesn’t mean as far away as the moon. It could be from an airplane or a low earth orbit satellite

That’s absolutely what Phil was talking about. In his show, and book, and on this blog, he talks about practical methods for diverting asteroids that we could (hypothetically) implement with today’s technology.

The “choice” we have that the dinosaurs didn’t is whether to just sit here blindly and wait for an asteroid to wipe us out (which in the very long term would be highly probable), or to do something about it and save ourselves by preventing the disaster.

Even if we could colonize other planets, that would still be the choice — let earth be hit, or do something about it. Why on earth would we not do something about it?!

Don Prothero, in ‘Catastrophes!’, thinks the Chicxulub impact is very much overrated as the cause of the extinction of the non-avian dinosaurs.

The non-avian dinosaurs were in decline before the impact. Some, such as certain Hadrosaursus species, lasted up to 1 million years after the impact, and certain classes of animals such as amphibians perplexedly sailed through the mass extinction.

Of course, if you were standing under the asteroid, it would really ruin your day. A decent sized impact would also be a civilization ender, even if it didn’t cause human extinction.

Don Prothero gives more credence to the Indian Deccan Traps supervolcano.

Mr Plait, regarding your note about the Sun’s orbit around the Milky Way potentially causing mass extinctions, I found a report comparing data between galactic spiral transit and terrestrial climate change:

ABSTRACT
We re-examine past suggestions of a close link between terrestrial climate change and the Sun’s transit of spiral arms in its path through the Milky Way galaxy. These links produced concrete fits, deriving the unknown spiral pattern speed from terrestrial climate correlations. We test these fits against new data on spiral structure based on CO data that do not make simplifying assumptions about symmetry and circular rotation. If we compare the times of these transits with changes in the climate of Earth, the claimed correlations not only disappear, but we also find that they cannot be resurrected for any reasonable pattern speed.

I know that ‘mass extinction’ and ‘climate change’ are not the same thing, though it is worth weighing this up and taking it into account.

This couldn’t have happened on early Earth, which explains why impactors are responsible for so few of the known extinction events. And such a pinpoint impact it is unlikely to happen again.

It is however true that, after the PFC ban saved the ozone layer, this is the 2nd preventable nature catastrophe we know. So we could work on it.

I don’t think there is much risk lowering ROI in it though. It is foremost a moral issue.

@ Wayne Robinson:

For the new science consensus, I believe the above paper from last year is informative. It lists some 20 scientists from different areas. The conclusion is that the K-Pg impactor was most likely responsible.

@ andy:

[affects finger swoop] I know! [/affects finger swoop]

It irks me very much when people suggest the dinosaurs went extinct. Even more when they compare to us, since by comparing diversification extant dinosaurs are more successful than extant mammals.

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* It is notable in this context that humans are amateurs in producing waste. There are many species that have produced more and riskier waste under geological times!

180 impact craters on Earth? Wow, my numbers were well out of date. I used to use the number of impact craters as an argument against Young Earth Creationism, but the only number I could find at the time was 60 craters. I pointed out that this would require one major impact every hundred years to accomplish on a 6000 year old Earth, which didn’t seem to jive well with all the craters being really old and none of them seeming to have occurred in human history.

180 impact craters would require an impact roughly every 30 years. Anybody remember one? You know, a really big one that left a lasting crater. Anyone?

A minor observation ~ in the topmost illustration, why is the object flaming? At that distance, it is passing through no atmosphere. Does that mean it is creating a roaring sound in the vacuum of space, as well? Just wondering.

Hey Phil, I got a question if you don’t mind. I am a grad student in particle physics and I got to deal with this whole Frequentist vs. Bayesian statistics debate occasionally, when it comes to limit settings or statistical significance estimation. I am wondering if you could tell me a bit more about how the Bayesian approach in this case is less biased than the Frequentist approach (which I assume, is what you mean when you talk about standard statistics.). I think this would help me understand a lot what’s currently going in my own field with the hunt for the Higgs and all that.

Since the planet is mostly covered in water with only about 30% dry land, it would suggest that more than twice as many impacts have occurred in the oceans. These no doubt generated humungous tsunamis and left no craters. Have these impacts been factored in?

@26
To quote #1 “How can an impact be so large it is only detected from space? Do you mean it is more easily (or first) detected from space? It seems like it’s always easier to detect things on Earth from Earth, than from space.”

Quoting #26
“Dr. Dreadful, you can detect the shape of North America by standing on it. You just have to stand on it in a bunch of different places, and do some geometry.”

Yes, standing in a LOT of places. It would be easier to look once from the ISS than to walk around the whole of North America to find it’s shape (if you ignore the work it takes to MAKE the ISS ).

…it wasn’t really going to hit us, or it would break up before it hit
…if it did hit us, it would be a good thing because it would bring new mineral resources
…trying to do something about it would cost jobs and destroy the economy
…meteorites have hit earth in the past
…it is all a big plot to bring about a one-world socialist government
…it is a ploy so government can expand their powers and take away your guns
…God is in heaven and in control and he won’t allow the world to be destroyed

For how long ? And also don’t forget that we invented not only spaceflight, but also denialism. And I would not bet on, that, when the time comes, we will launch an asteroid deflecting mission, and not an asteroid impact denialism propaganda campaign of astronomic proportions. I certainly hope that the former will be the case, but I wouldn’t bet on it.

Wayne #22: Don Prothero just rehashes old long discredited arguments. Bad statistics on dating of fossils that don’t take into account how error bars on fossil dating will cause a sudden extinction event to look like the extinction occurred over a period of time. He uses the same arguments that were discussed 20-30 years ago and packages them to sound as if they were brand new.

Messier, there have been a number of stars that passed our “neighborhood” since the solar system was formed, some DID dislodge Oort cloud material into the inner solar system and some did impact, to judge from the sparse evidence.
That said, I DO disparage the view that ONE impactor killed off the dinosaurs. If such an extreme, as has frequently been parroted had happened, kindly explain the survival of both the delicate frogs AND BIRDS?
Now, an eruption of the Deccan Traps, during or shortly after, a MULTIPLE, which is more probable, some tidal effect disrupting the main body thing going on, impact, that generates the infamous crater AND Shiva and possibly a few others, COULD explain how dinosaurs died off, much of plant life died off and frogs and birds survived.
No incinerator atmosphere, else birds are toasted nicely out of the universe. Frogs wouldn’t fare much better and FORGET coral. The acid rain would extinct THAT *REAL* quick.
No, the evidence backs the models of the animals in question and a handful more, ONE shock event would kill the lot in PREFERENCE to the larger dinosaurs. A collection of “bad days” over time WOULD account for the mass extinction, leaving delicate members here and there to survive into today.
Besides, I’ve YET to hear a decent explanation for the massive amounts of iridium in the KT boundary in India, as in a METER MORE than the REST of the globe.
Still, we DO need to study those rocks and rubble heaps and figure GOOD plans to relocate them into a better neighborhood, rather than our “streets”.

The Moon has way more craters preserved than the Earth does. On average, it has our orbit around the Sun. At the moment, lunar craters can be aged using crater counts. LRO is providing data on tiny craters. There’s a Zooniverse project on it… There’s a bit of ground truth from Apollo. So, it looks like the Moon is a better place to do statistics.

#45
There’s evidence pondered which implies an insect blood-borne illness may have exacerbated the demise of any dinosaurs who managed to survive the effects of a gigantic asteroid impact. I saw this on a science program a long time ago and the details are foggy, but I think that’s the gist of it. It’s not necessarily a single punch that deals the death blow, but more likely a combination of punches that overwhelm and permanently knock out a species, of which our own species is not immune.

Geri #43: there are indications that the KT impact itself isn’t the whole story. Analysis of zooid sizes in bryozoans indicates that there were significant temperature spikes preceding the impact by 100,000’s years, which probably stressed many ecosystems. There are also 8 species of ammonite from two different genera known to occur above the Iridium layer in northern Europe; they have been confirmed not to be reworked and sit in layers about 200,000 above the Iridium layer.

Comment 41 by Daniel Andrews is right on the mark. I also agree that most impacts would occur ar sea. One the size of say, 2 aircraft carriers, would cause a huge mess on land but no big deal in the ocean. If Tungaska had been at sea we wouldn’t know it had happened.